1. Field of the Invention
This invention relates to an insulated wire useful as a magnet wire for use in electric equipment such as motors, transformers and the like, and to a process for producing the insulated wire.
2. Description of the Prior Art
Most of the conventional magnet wires insulated with an insulating varnish heretofore in use generally do not satisfy all the critical properties recently required, except for magnet wires coated with recently developed heat resistant resins, such as the polyamideimides, and techniques for multiple coating of the conductor with two or more different insulating materials have been proposed as disclosed in U.S. Pat. No. 3,022,200, U.S. Pat. No. 3,702,813, etc.
Generally speaking, the multiple coating insulating layer exhibits some advantages, i.e., the excellent properties possessed by specific layers included in the layers of multiple coatings compensate for the defects of the other layers. On the other hand, the excellent properties possessed by the layers sometimes can be masked by the presence of other layers and, in some cases, the presence of other layers adversely affects the properties. For example, the difficulty of operations for uncovering the tip of the wire coated with a polyamide-imide resin due to an excessively strong chemical resistance of the resin to a remover can be solved by providing a polyester layer under the polyamide-imide layer, but, due to the presence of the polyester layer the abrasion resistance, the heat resistance, etc., of the polyester-polyamide-imide dual coated wire are inferior to those of a wire coated with the polyamide-imide alone. The thermoplastic polyester layer of the insulated wire wherein the thermoplastic polyester layer is coated on a baked thermosetting polyester layer functions to improve the heat-shock resistance and the abrasion resistance properties of the baked polyester layer, but it lowers the cut through temperature of the insulated wire.
As a result of extensive studies, it was surprisingly found that a dual insulating coating comprising a specific polyacryl resin layer having coated thereon a specific polyester resin layer exhibits excellent heat resistance properties over a coating of each of the above described resins and further it possesses well-balanced overall properties required for a coating of magnet wires. The present invention has been completed by a further investigation of the above unexpected finding.